To provide an electronic ignition for internal combustion engines, such as Otto cycle (i.e., four-stroke) engines, circuits with a switch, a spark plug, and a transformer connected across the spark plug and switch may be used. Typically, the switch may be implemented by a solid-state component, such as an IGBT transistor or Darlington configuration bipolar transistors (i.e., a Darlington configuration transistor).
Electronically closing the solid-state switch using a control signal allows current to flow through the primary winding of a transformer that charges with an energy equal to L*I2/2. The variable L is the inductance value of the primary winding, and the variable I is the current flowing through it.
Once the primary winding has been charged to the desired energy necessary to generate a spark at the secondary winding, the solid-state switch is electronically opened. After opening the solid-state switch, the current at the primary winding is abruptly interrupted and the energy, previously stored in it, is released as a voltage pulse across the primary winding. Such a pulse may be equal to 200-400 Volts, for example.
The voltage variation across the primary winding generates an electric field, and therefore, there is a mutual inductance coupling with the secondary winding. The secondary winding is sized, for example, with a turns ratio N2/N1 of about 100. A voltage is generated in the secondary winding, which is about 100 times the voltage in the primary winding, and is about 20-40 kV. This voltage is applied to the spark plug connected to the secondary winding of the transformer. This is sufficient to generate a spark in the combustion chamber in which the spark is received.
If a plug is not present or does not perfectly operate when the solid-state switch is opened, a condition known as an open secondary occurs. The energy stored in the primary of the transformer cannot be usefully transferred to the transformer secondary. Consequently, the energy is dissipated in the primary winding. More particularly, the energy will be dissipated by the solid-state component operating as a switch.
This condition is particularly demanding for a solid-state component when the device operates in a current limiting condition. The current is a maximum current allowed by the ignition circuit configuration, and therefore, the energy to be dissipated takes a maximum value.
In addition, the problem of power dissipation from the solid-state switch is further exacerbated because the marketplace requires devices which are increasingly smaller in terms of their silicon and package areas. These factors reduce the thermal capacities instrumental to the dissipation process in case of an open secondary.
In light of what has been described beforehand regarding the thermal dissipation problems in the open secondary condition, this condition is a potential cause of breakdown of the electronic ignition circuit. The breakdown may be the solid-state power element acting as the switch, along with consequential damages of the portions which the electronic ignition circuit is connected to, such as the coil and the electronic unit, for example.
U.S. Published Patent Application No. 2004/0200463 describes a device for the ignition of internal combustion engines having, among other things, an IGBT transistor acting as a switch, a current limiting circuit, and an anomalies detecting circuit capable of detecting an anomaly in the ignition signal generated by an electronic control unit.